* A very dirty hack to make setuid installations a bit nicer to use.

Previously there was the problem that all files read by nix-env
  etc. should be reachable and readable by the Nix user.  So for
  instance building a Nix expression in your home directory meant that
  the home directory should have at least g+x or o+x permission so
  that the Nix user could reach the Nix expression.  Now we just
  switch back to the original user just prior to reading sources and
  the like.  The places where this happens are somewhat arbitrary,
  however.  Any scope that has a live SwitchToOriginalUser object in
  it is executed as the original user.

* Back out r1385.  setreuid() sets the saved uid to the new
  real/effective uid, which prevents us from switching back to the
  original uid.  setresuid() doesn't have this problem (although the
  manpage has a bug: specifying -1 for the saved uid doesn't leave it
  unchanged; an explicit value must be specified).

configure.ac

@@ -151,10 +151,10 @@ if test "$setuid_hack" = "yes"; then
     AC_DEFINE(SETUID_HACK, 1, [whether to install Nix setuid])
 fi
 
-AC_CHECK_FUNC(setreuid, [HAVE_SETREUID=1], [HAVE_SETREUID=])
-AM_CONDITIONAL(HAVE_SETREUID, test "$HAVE_SETREUID" = "1")
-if test "$HAVE_SETREUID" = "1"; then
-    AC_DEFINE(HAVE_SETREUID, 1, [whether we have setreuid()])
+AC_CHECK_FUNC(setresuid, [HAVE_SETRESUID=1], [HAVE_SETRESUID=])
+AM_CONDITIONAL(HAVE_SETRESUID, test "$HAVE_SETRESUID" = "1")
+if test "$HAVE_SETRESUID" = "1"; then
+    AC_DEFINE(HAVE_SETRESUID, 1, [whether we have setresuid()])
 fi
 
 AC_ARG_WITH(nix-user, AC_HELP_STRING([--with-nix-user=USER],

src/Makefile.am

@@ -4,7 +4,7 @@ SUBDIRS = bin2c boost libutil libstore libmain nix-store nix-hash \
 SETUID_PROGS = nix-store nix-instantiate nix-env
 install-exec-hook:
 if SETUID_HACK
-if HAVE_SETREUID
+if HAVE_SETRESUID
 	cd $(DESTDIR)$(bindir) && chown @NIX_USER@ $(SETUID_PROGS) \
 	  && chgrp @NIX_GROUP@ $(SETUID_PROGS) && chmod ug+s $(SETUID_PROGS)
 else

src/libexpr/parser.cc

@@ -106,6 +106,8 @@ static Expr parse(EvalState & state,
 
 Expr parseExprFromFile(EvalState & state, Path path)
 {
+    SwitchToOriginalUser sw;
+
     assert(path[0] == '/');
 
 #if 0

src/libmain/shared.cc

@@ -169,9 +169,14 @@ static void initAndRun(int argc, char * * argv)
 }
 
 
-#if HAVE_SETREUID
-#define _setuid(uid) setreuid(uid, uid)
-#define _setgid(gid) setregid(gid, gid)
+static bool haveSwitched;
+static uid_t savedUid, nixUid;
+static gid_t savedGid, nixGid;
+
+
+#if HAVE_SETRESUID
+#define _setuid(uid) setresuid(uid, uid, savedUid)
+#define _setgid(gid) setresgid(gid, gid, savedGid)
 #else
 /* Only works properly when run by root. */
 #define _setuid(uid) setuid(uid)
@@ -179,6 +184,37 @@ static void initAndRun(int argc, char * * argv)
 #endif
 
 
+SwitchToOriginalUser::SwitchToOriginalUser()
+{
+#if SETUID_HACK && HAVE_SETRESUID
+    /* Temporarily switch the effective uid/gid back to the saved
+       uid/gid (which is the uid/gid of the user that executed the Nix
+       program; it's *not* the real uid/gid, since we changed that to
+       the Nix user in switchToNixUser()). */
+    if (haveSwitched) {
+        if (setuid(savedUid) == -1)
+            throw SysError(format("temporarily restoring uid to `%1%'") % savedUid); 
+        if (setgid(savedGid) == -1)
+            throw SysError(format("temporarily restoring gid to `%1%'") % savedGid); 
+    }
+#endif
+}
+
+
+SwitchToOriginalUser::~SwitchToOriginalUser()
+{
+#if SETUID_HACK && HAVE_SETRESUID
+    /* Switch the effective uid/gid back to the Nix user. */
+    if (haveSwitched) {
+        if (setuid(nixUid) == -1)
+            throw SysError(format("restoring uid to `%1%'") % nixUid); 
+        if (setgid(nixGid) == -1)
+            throw SysError(format("restoring gid to `%1%'") % nixGid); 
+    }
+#endif
+}
+
+
 void switchToNixUser()
 {
 #if SETUID_HACK
@@ -208,7 +244,7 @@ void switchToNixUser()
     /* !!! Apparently it is unspecified whether getgroups() includes
        the effective gid.  In that case the following test is always
        true *if* the program is installed setgid (which we do when we
-       have setreuid()).  On Linux this doesn't appear to be the
+       have setresuid()).  On Linux this doesn't appear to be the
        case, but we should switch to the real gid before doing this
        test, and then switch back to the saved gid. */ 
 
@@ -227,13 +263,14 @@ void switchToNixUser()
         return;
     }
 
+    savedUid = getuid();
+    savedGid = getgid();
+
     /* Set the real, effective and saved gids to gr->gr_gid.  Also
        make very sure that this succeeded.  We switch the gid first
        because we cannot do it after we have dropped root uid. */
-    if (_setgid(gr->gr_gid) != 0 ||
-        getgid() != gr->gr_gid ||
-        getegid() != gr->gr_gid)
-    {
+    nixGid = gr->gr_gid;
+    if (_setgid(nixGid) != 0 || getgid() != nixGid || getegid() != nixGid) {
         cerr << format("unable to set gid to `%1%'\n") % NIX_GROUP;
         exit(1);
     }
@@ -248,14 +285,14 @@ void switchToNixUser()
     /* This will drop all root privileges, setting the real, effective
        and saved uids to pw->pw_uid.  Also make very sure that this
        succeeded.*/
-    if (_setuid(pw->pw_uid) != 0 ||
-        getuid() != pw->pw_uid ||
-        geteuid() != pw->pw_uid)
-    {
+    nixUid = pw->pw_uid;
+    if (_setuid(nixUid) != 0 || getuid() != nixUid || geteuid() != nixUid) {
         cerr << format("unable to set uid to `%1%'\n") % NIX_USER;
         exit(1);
     }
 
+    haveSwitched = true;
+    
 #endif
 }
 

src/libstore/store.cc

@@ -157,7 +157,10 @@ void copyPath(const Path & src, const Path & dst)
     
     CopySink sink;
     sink.fd = pipe.writeSide;
-    dumpPath(src, sink);
+    {
+        SwitchToOriginalUser sw;
+        dumpPath(src, sink);
+    }
 
     /* Wait for the child to finish. */
     int status = pid.wait(true);
@@ -421,7 +424,11 @@ Path addToStore(const Path & _srcPath)
     Path srcPath(absPath(_srcPath));
     debug(format("adding `%1%' to the store") % srcPath);
 
-    Hash h = hashPath(srcPath);
+    Hash h;
+    {
+        SwitchToOriginalUser sw;
+        h = hashPath(srcPath);
+    }
 
     string baseName = baseNameOf(srcPath);
     Path dstPath = canonPath(nixStore + "/" + (string) h + "-" + baseName);

src/libutil/util.hh

@@ -246,4 +246,14 @@ string statusToString(int status);
 bool statusOk(int status);
 
 
+/* !!! HACK HACK HACK - this should be in shared.hh, but it's to
+   facilitate a quick hack - will remove this eventually (famous last
+   words). */
+struct SwitchToOriginalUser
+{
+    SwitchToOriginalUser();
+    ~SwitchToOriginalUser();
+};
+
+
 #endif /* !__UTIL_H */

src/nix-env/main.cc

@@ -600,6 +600,7 @@ static void opSwitchProfile(Globals & globals,
     Path profile = opArgs.front();
     Path profileLink = getHomeDir() + "/.nix-profile";
 
+    SwitchToOriginalUser sw;
     switchLink(profileLink, profile);
 }
 
@@ -698,6 +699,7 @@ static void opDefaultExpr(Globals & globals,
     Path defNixExpr = absPath(opArgs.front());
     Path defNixExprLink = getDefNixExprPath();
     
+    SwitchToOriginalUser sw;
     switchLink(defNixExprLink, defNixExpr);
 }
 
@@ -772,6 +774,7 @@ void run(Strings args)
     if (!op) throw UsageError("no operation specified");
 
     if (globals.profile == "") {
+        SwitchToOriginalUser sw;
         Path profileLink = getHomeDir() + "/.nix-profile";
         globals.profile = pathExists(profileLink)
             ? absPath(readLink(profileLink), dirOf(profileLink))